Create (and close) a new socket in rpc Connect, don't reuse one.

It appears that connecting more than one with the same nse_nsock socket
leaks socket descriptor. For example,

local s = nmap.new_socket()
s:connect(host, port) --> TIMEOUT
s:connect(host, port) --> TIMEOUT
s:close()

leaks a socket descriptor, the one used in the first connect. Nsock
should really take care of this, but let's do this workaround because
rpc-grind has been causing problems due to using the above pattern:

http://seclists.org/nmap-dev/2012/q3/864
http://seclists.org/nmap-dev/2012/q3/872
http://seclists.org/nmap-dev/2012/q3/949

The difficulty is that the rpc library will tolerate around 400 of those
timeouts per RPC connection, which leads to rapidly running out of
descriptors.

nselib/rpc.lua

@@ -164,26 +164,28 @@ Comm = {
         return status, err
       end
       if ( port.protocol == "tcp" ) then
-        socket = nmap.new_socket()
         if nmap.is_privileged() then
           -- Try to bind to a reserved port
           for resvport = 600, 1024, 1 do
+            socket = nmap.new_socket()
             status, err = socket:bind(nil, resvport)
             if status then 
               status, err = socket:connect(host, port)
               if status then break end
+              socket:close()
             end
           end
         else
           status, err = socket:connect(host, port)
         end
       else
-        socket = nmap.new_socket("udp")
         if nmap.is_privileged() then
           -- Try to bind to a reserved port
           for resvport = 600, 1024, 1 do
+            socket = nmap.new_socket("udp")
             status, err = socket:bind(nil, resvport)
             if status then break end
+            socket:close()
           end
         end
       end